High performance heat exchangers

ABSTRACT

Disclosed are means for improving the service-life of indirect tubesheet type heat exchangers used in chemical reactors, particularly those exposed to reducing, nitridizing and/or carburizing environments. Such means include the use of certain ferrules within the heat exchange tubes and/or weld types used in construction of these heat exchangers.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to means for improving theservice-life and reliability of indirect heat exchangers used inchemical reaction systems which produce high temperature, reactiveeffluent gasses. In particular, this invention relates to means forimproving service-life of indirect heat exchangers used in theproduction of hydrogen cyanide.

[0002] In many chemical processes, the reaction effluent comprises hot,reactive, and/or abrasive fluids and/or gasses. For many of thesereactions, it is desirable to rapidly quench the reactor effluent toprevent decomposition of the product component(s). Quenching may beaccomplished through direct contact, such as application of watersprays, or more commonly through indirect methods such as through theuse of indirect heat exchangers. Because indirect heat exchangersprovide the added advantage that they may be configured to recover wasteheat, these are the more preferred method and have been in use for manyyears. Typical indirect heat exchangers used in such chemical processesconsist of a shell and tube design.

[0003] In many systems, the thermal, kinetic and reactive properties ofthe effluent may individually or collectively serve to corrode, crack,or otherwise degrade the materials used to form the heat exchange zone.In particular, the exchange tubes and the tube-to-tubesheet weldsnearest to the reaction zone see the most severe conditions and are mostsusceptible to degradation. For example, the hot effluent may chemicallyreact with the metal of the heat exchange tube itself, thereby causingerosion and/or corrosion, that is, metal dusting, carburization and thelike, all of which lead to failure of the heat exchanger. The weld areaof the heat exchange tubes is also susceptible to stress corrosioncracking, which leads to failure of the heat exchanger. These problemsmay be encountered, for example, in the production of hydrogen cyanideor acrylonitrile, in nitric acid waste heat recovery exchangers, inhydrocarbon cracking units, and in tubeside fired boilers andexchangers.

[0004] The use of ferrules as a protective covering for tubes in shelland tube heat exchangers and condensers is well known. Such ferrulesprimarily insulate the tubes and welds but also protect the heatexchange tubes against deterioration resulting from chemical attack.Typically, heat exchangers are placed downstream of a reaction zone in achemical reactor, such as downstream of a catalyst. Thus, the top orupper portion of the heat exchange tubes are exposed to hot effluentgasses. Certain ferrules, such as alumina, are employed to provideinsulation against the heat from such gasses.

[0005] In a common application, an exchanger tube is first constructedfrom a base material; a second, possibly different material, selectedfor its properties, such as resistance to erosion and/or chemicalattack, is then formed into a tube that slides inside the inlet/upstreamend of the heat exchanger tube. Depending on the severity of the processconditions, the ferrules may provide a long period of maintenance-freeservice or alternatively, they may be sacrificial, requiring frequentreplacement. In either case, the use of the ferrules provides aneconomical method for extending the service life of the base-materialexchanger through the prevention of erosion and/or corrosion.

[0006] For example, M. James, “Unexpected Metal Dusting Failure of WasteHeat Boiler Tubes”, First International Symposium on InnovativeApproaches for Improving Heat Exchanger Reliability, Proceedings,Materials Technology Institute of the Chemical Process Industries, Inc.,1-13 (1998) discloses a specific ceramic ferrule design in a shell andtube reactor for use in a chemical process having reducing, carburizingand/or nitridizing conditions. In this paper, various nickel-chromiumalloys (INCONEL) were used in the tubes of a heat exchanger. In eachcase, the INCONEL material experienced severe wastage, that is, metaldusting, at rates much higher than known rates.

[0007] Also, U.S. Pat. No. 5,775,269 discloses a boiler protection tubeassembly having an inner ceramic sleeve, a ceramic block and an outerceramic sleeve.

[0008] The ceramics disclosed in U.S. Pat. No. 5,775,269 are aluminumand zirconium oxides. Such ceramics are impractical for use in heatexchangers requiring rapid quenching of very hot effluent as thesematerials last only a relatively short time under such temperatureextremes.

[0009] Ceramics, such as alumina, silica and zirconia, are effective asinsulators in such reactors as steam-methane reformers. However, theysuffer from poor thermal shock resistance and may, in the case ofsilica, react with hydrogen, which is present in many reducingenvironments. See, for example, M. S. Crowley, Hydrogen-Silica Reactionsin Refractories, Ceramic Bulletin, Vol. 46, No. 7, 679-682 (1967). Thus,these ceramic materials are unsuitable for use in chemical processesrequiring rapid quenching of hot effluent gasses and/or in reducingenvironments, both of which are found in the production of hydrogencyanide.

[0010] In a typical shell and tube reactor, the heat exchange tubes areattached to a tubesheet at each tube end. Typically, a tube is passedthrough a hole in the tubesheet until the end of the tube isapproximately flush with the top surface of the tubesheet. The tube isthen typically welded to the top surface of the tubesheet. Generally,the outer diameter of the tube is smaller than the inner diameter of thecorresponding hole in the tubesheet. Thus, once the tube is welded tothe tubesheet, an annular space remains between the tube and thetubesheet below the weld. FIG. 1a shows a typical weld 3 used in shelland tube reactors, especially reactors used for chemical processeshaving reducing environments, such as in the production of hydrogencyanide. The tubes may also be affixed to the tubesheet by alternatemeans, such as rolling. FIG. 1b shows a typical attachment of a tube toa tubesheet by rolling.

[0011] In hydrogen cyanide production, the hot effluent gasses must berapidly cooled from about 1000° to 1400° C. to about 600° C. or less inorder to prevent decomposition of the hydrogen cyanide. As such effluentgasses are cooled, the tubesheet, weld and upper portion of the exchangetubes become very hot. As a result, any water present in the annularspace 5 vaporizes and deposits any impurities contained in the water inthe annular space 5. Such impurities typically are ions and minerals inthe water and the like. Such impurities are also typically corrosive tothe tubesheet, exchange tube, and particularly the weld. Over time, suchcorrosive materials buildup in the annular space 5. The combination ofheat from the effluent gasses and corrosive materials with stresses inthe system leads to stress corrosion cracking in the tube, weld and/ortubesheet. Such stress corrosion cracking leads to failure, andultimately replacement, of the heat exchanger.

[0012] A number of shell and tube hydrogen cyanide reactor designs havebeen developed to address the problem of minimizing the heat thetubesheet, exchange tubes and weld are exposed to. FIGS. 3a-e illustratesuch reactors.

[0013] Each reactor is designed with high cooling water flow rates,turbulent cooling water flow, and refractory to insulate the tubesheet.However, these designs do not completely prevent such stress corrosioncracking.

[0014] Down-hole welds, or full penetration welds, have been used inchemical processes that do not have reducing environments or such rapidquenching requirements. For example, Ahmed et al., Failure, Repair andReplacement of Waste Heat Boiler, Ammonia Plant Safety & RelatedFacilities, American Institute of Chemical Engineers, Vol. 37, 100-110,discloses the use of such a weld in a horizontal heat exchanger for usein a secondary ammonia reformer. This paper does not disclose the use ofthis weld for any other process.

[0015] Thus, the problem of providing effective heat exchangers having along-service life in shell and tube reactors used in chemical processeshaving reducing environments remains.

SUMMARY OF THE INVENTION

[0016] It has been surprisingly found that the service life of heatexchangers in shell and tube hydrogen cyanide reactors can be extendedby using a ferrule, particularly a ferrule including silicon nitride,and/or by using a down-hole weld to attach the heat exchange tubes tothe tubesheet.

[0017] In one aspect, the present invention is directed to a heatexchange apparatus for use in a reducing, carburizing and/or nitridizingenvironment including: (a) a shell having an entry tubesheet portion andan exit tubesheet portion, each tubesheet having a plurality of holes,wherein the shell has at least one inlet and one outlet for heatexchange medium; (b) a plurality of tubes disposed within the shellwherein an entry end of each tube is affixed to the entry tubesheet andan exit end of each tube is affixed to the exit tubesheet such that anaxis of the tube and an axis of an entry and exit tubesheet hole arecoincident; and (c) a plurality of ferrules, each ferrule having anentry end and an exit end extending through an entry tubesheet hole intoa tube wherein the exit end extends below the entry tubesheet, theferrule including silicon nitride.

[0018] In a second aspect, the present invention is directed to a heatexchange apparatus including: (a) a shell having an entry tubesheetportion and an exit tubesheet portion, each tubesheet having a pluralityof holes, wherein the shell has at least one inlet and one outlet forheat exchange medium; (b) a plurality of tubes disposed within the shellwherein an entry end of each tube is affixed to the entry tubesheet andan exit end of each tube is affixed to the exit tubesheet such that anaxis of the tube and an axis of an entry and exit tubesheet hole arecoincident, each tube being formed of a metal including nickel-chromiumalloy; and (c) a plurality of ferrules, each ferrule having an entry endand an exit end extending through an entry tubesheet hole into a tubewherein the exit end extends below the entry tubesheet, the ferruleincluding nickel-chromium alloy.

[0019] In a third aspect, the present invention is directed to a ferrulefor use in a heat exchange tube wherein the ferrule has an entry end andan exit end; the entry end having an opening tapering conically into apipe section, the outer diameter of the entry end being greater than aninner diameter of the heat exchange tube; the pipe section having anouter diameter up to 99% of the inner diameter of the heat exchangetube; the pipe section having an expanded area with an outer diameterthat is substantially the same as the inner diameter of the heatexchange tube.

[0020] In a fourth aspect, the present invention is directed to aferrule for use in a heat exchange tube wherein the ferrule has an entryend and an exit end; the entry end having an opening tapering conicallyor trumpet-shaped into a pipe section, the outer diameter of the entryend being greater than an inner diameter of the heat exchange tube; thepipe section having an outer diameter that is substantially the same asthe inner diameter of the heat exchange tube; and wherein the ferrulehas a venturi-shaped design in longitudinal cross-section.

[0021] In a fifth aspect, the present invention is directed to a heatexchange apparatus for use in a hydrogen cyanide reactor including: (a)a shell having an entry tubesheet portion and an exit tubesheet portion,each tubesheet having a plurality of holes, wherein the shell has atleast one inlet and one outlet for heat exchange medium; and (b) aplurality of tubes disposed within the shell wherein an entry end ofeach tube is affixed to the entry tubesheet and an exit end of each tubeis affixed to the exit tubesheet such that an axis of the tube and anaxis of an entry and exit tubesheet hole are coincident, wherein eachtube entry end is affixed to the entry tubesheet by a down-hole weld.

[0022] In a sixth aspect, the present invention is directed to anapparatus for preparing hydrogen cyanide by reacting hydrocarbon,ammonia and optionally oxygen-containing gas in the presence of aplatinum-containing catalyst at a temperature in the range from 1000° to1400° C., including a reaction zone, an optional refractory zone, and aheat exchange zone including: (a) a shell having an entry tubesheetportion and an exit tubesheet portion, each tubesheet having a pluralityof holes, wherein the shell has at least one inlet and one outlet forheat exchange medium; (b) a plurality of tubes disposed within the shellwherein an entry end of each tube is affixed to the entry tubesheet andan exit end of each tube is affixed to the exit tubesheet such that anaxis of the tube and an axis of an entry and exit tubesheet hole arecoincident; and (c) a plurality of ferrules, each ferrule having anentry end and an exit end extending through an entry tubesheet hole intoa tube wherein the exit end extends below the entry tubesheet, theferrule including silicon nitride.

[0023] In a seventh aspect, the present invention is directed to aprocess for preparing hydrogen cyanide including the steps of: feedingreaction gas to a reactor, the reaction gas including hydrocarbon,ammonia and optionally an oxygen-containing gas; reacting the reactiongas in the presence of a catalyst to give product gas; cooling theproduct gas in a heat exchange apparatus including (a) a shell having anentry tubesheet portion and an exit tubesheet portion, each tubesheethaving a plurality of holes, wherein the shell has at least one inletand one outlet for heat exchange medium; (b) a plurality of tubesdisposed within the shell wherein an entry end of each tube is affixedto the entry tubesheet and an exit end of each tube is affixed to theexit tubesheet such that an axis of the tube and an axis of an entry andexit tubesheet hole are coincident; and (c) a plurality of ferrules,each ferrule having an entry end and an exit end extending through anentry tubesheet hole into a tube wherein the exit end extends below theentry tubesheet, the ferrule including silicon nitride; and recoveringhydrogen cyanide from the cooled product gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1a shows a schematic cross-section of a typical exchange tubeto tubesheet weld used in hydrogen cyanide reactors having anexaggerated annular space.

[0025]FIG. 1b shows a schematic cross-section of a rolled exchange tubeto a tubesheet connection having an exaggerated annular space.

[0026]FIG. 2a shows a schematic cross-section of a tubesheet andexchange tube affixed by a down-hole weld having an exaggerated annularspace.

[0027]FIG. 2b shows a schematic cross-section of a tubesheet andexchange tube situated for a down-hole weld having an exaggeratedannular space.

[0028]FIGS. 3a to 3 e show schematic cross-sections hydrogen cyanidereactor heat exchanger designs.

[0029]FIG. 4a shows a schematic cross-section of a ferrule providing aninsulating gas space.

[0030]FIG. 4b shows a schematic cross-section of a ferrule having aconical entry end and an expanded diameter in one section of the pipe.

[0031]FIG. 5 shows a schematic cross-section of a ferrule having aconical entry end and a straight exit end.

[0032]FIG. 6 shows a schematic cross-section of a ferrule having aconverging/diverging design.

[0033]FIG. 7 shows a schematic cross-section of a ferrule having astraight bore.

[0034]FIG. 8 shows a schematic cross-section of a shell and tube heatexchanger including ferrules and ferrule sleeves.

[0035]FIG. 9 shows a schematic cross-section of a two-ferrule systemwith a ferrule sleeve in a heat exchange tube.

DETAILED DESCRIPTION OF THE INVENTION

[0036] As used throughout the specification, the following terms shallhave the following meanings, unless the context clearly indicatesotherwise. The terms “heat exchange units” and “heat exchange vessels”are used interchangeably. The terms “first” and “second” and “upper” and“lower” and “entry and exit”, respectively, are used interchangeably,though it is recognized that the reactor may be in a vertical,horizontal, or other configuration in which the terms “upper” and“lower” would not be apt to describe the relation of components to oneanother. The term “entry” refers to that portion of the heat exchangeapparatus that is nearest to the reaction zone, that is, where hotprocess gasses enter the heat exchange apparatus. The term “exit” refersto that portion of the heat exchange apparatus that is furthest from thereaction zone, that is, where hot process gasses exit the heat exchangeapparatus. The following abbreviations are used throughout thespecification: C=centigrade and cm=centimeter. All ratios and amountsare by weight, unless otherwise noted. All numerical ranges areinclusive, unless otherwise noted.

[0037] The present invention provides a solution to the problem of shortservice-life of shell and tube heat exchangers through the use ofspecific ferrules and/or welds. Preferably, the design of the ferrulesof the present invention further serves to minimize the potentiallydamaging effects of the reactor effluent stream. In addition, the designof the heat exchanger and the use of down-hole welds in affixing theexchange tubes to the tubesheet of the apparatus simplify construction,maintenance and repair of the unit, minimizing downtime and increasingoperational efficiency. The down-hole weld also eliminates the annularspace between the exchange tube and the tubesheet which can serve as acrevice to concentrate corrosive agents. Thus the present invention isuseful in extending the service-life of any shell and tube heatexchanger. The ferrules and welds of the present invention areparticularly useful in shell and tube heat exchangers that are exposedto hot, reactive effluent gasses, and preferably where the heatexchanger is exposed to a reducing, carburizing or nitridizingenvironment, such as in hydrogen cyanide production.

[0038] There are numerous heat exchanger designs suitable for use withreactors producing reactive, high temperature, high abrasion effluentstreams. Particularly in connection with hydrogen cyanide production,the need for quenching the hot process gasses has long been appreciated.By quenching the hot process or effluent gasses is meant cooling thegasses sufficiently such that the products contained therein do notdegrade. For example, it is well known in hydrogen cyanide productionthat if the hot process gasses are not cooled sufficiently, the hydrogencyanide present will degrade.

[0039] Typical heat exchange units, or vessels, useful in hydrogencyanide production are illustrated in FIGS. 3a to 3 e. These units havethe following elements which are identified by like number in eachfigure: hot process gas inlets 1, cooled process gas outlets 6,exchanger cooling water inlet 7, exchanger cooling water outlet 8,exchanger tubesheet vent 9, tubesheet 4, heat exchange tubes 2, heatexchange vessel 10, reaction vessel 11, and refractory material 12. Therefractory material 12 shields the tubesheet from direct exposure to thehot process gasses. Arrows indicate the direction of flow of gasses orcooling water.

[0040] The unit of FIG. 3a is configured such that the lower portion ofthe reactor vessel extends into and is surrounded by the heat exchangevessel. This allows the cooling water outlets to be positioned above thelevel of the junction of the exchange tubes and the tubesheet. Thetubesheet forms a convex interior upper surface of the heat exchangevessel. The process gasses flow down and through the exchange tubeswhich are surrounded by the cooling water. The exchange tubes passthrough the tubesheet and are welded to the tubesheet using conventionalwelds at the interface between the upper lip of the exchange tube andthe upper surface of the tubesheet. The heated water and bubbles formedtherein during the quenching of the hot process gasses are generallydirected away from the exchange tubes to the cooling water outlets.

[0041] The unit of FIG. 3b has a planar tubesheet, generally of arelatively large thickness. The process gasses pass through thetubesheet and down into exchange tubes. Cooling water enters the heatexchange vessel from the bottom and primarily exits through a coolingwater outlet in the upper portion of the vessel. The tubesheet containsintegral channels that allow cooling water to flow through ithorizontally. There are also exchanger tubesheet vents positioned at thetop of the vessel, above the main outlet, which receive the water fromthe tubesheet channels. These vents are designed to remove the hottestwater and bubbles entrained therein.

[0042] The unit of FIG. 3c utilizes the reverse orientation for the exitflow of the heat exchange medium. The tubesheet is in the form of anannulus with the exchange tubes extending down therefrom. The coolingwater outlet is in the center of the annulus and carries the heatedwater up and out of the heat exchange vessel through center of thereactor vessel. The cooling water inlets are at the bottom of the heatexchange vessel. The upward flow of the cooling water, and thepreferable slope of the tubesheet, angled upward toward the coolingwater outlet promotes the removal of the heated water and entrainedbubbles away from the tubesheet and the exchanger tubes.

[0043] The unit of FIG. 3d provides a tubesheet with a planar uppersurface and which forms a concave upper surface of the heat exchangervessel. The cooling water enters the heat exchange vessel toward theupper portion of the vessel and exits through the cooling water outletnear the base of the vessel. The positioning of the inlet and outlet,and the concave roof of the vessel tend to direct the coolest watertoward the lower surface of the tubesheet and the exchanger tubesemerging from the tubesheet. Exchanger tubesheet vents with outletspositioned near the apex of the vessel serve to remove the heated waterand bubbles away from the tubesheet and exchange tubes.

[0044] In each of the units illustrated in FIGS. 3a to 3 d, the processgasses flow downward through the units. In the unit of FIG. 3e, theprocess gasses flow upward. In this orientation, the reaction vessel isbelow the heat exchange vessel. The gasses flow upward past therefractory material through the tubesheet and into the exchange tubes.Cooling water enters the heat exchange vessel at the bottom of thatvessel just above the tubesheet, and is removed from an outlet near thetop of that vessel. As a result, the heated water and gas bubbles, suchas vaporized water, rise to the top of the vessel, that is, away fromthe tubesheet. However, solids in the cooling water, especiallyminerals, may precipitate and accumulate on the exposed upper surface ofthe tubesheet. Such mineral deposits create localized hotspots whichpromote degradation of the tubesheet, the exchange tubes and tubesheetwelds adjacent thereto. An advantage of this design is that theformation of a gas layer adjacent to the tubesheet, that is a barrier toheat exchange, is minimized.

[0045] The cooling efficiency of shell and tube heat exchangers, such asthose in FIGS. 3a to 3 e, may be improved by the use of baffles withinthe shell. Such baffles direct the flow of the heat exchange mediumwithin the shell. The size, shape and placement of the baffles withinthe shell are specific to the particular heat exchanger design employed.Such baffle design and placement is within the ability of one skilled inthe art and such baffles may allow the water inlet(s) and outlet(s) tobe reversed if desired. It will be appreciated that more than one heatexchanger may be connected in series, which may also increase thecooling of the reactor effluent.

[0046] In shell and tube heat exchangers, the tubesheets are typicallyfrom about one-eighth of an inch (0.3 cm) to about twenty inches (50 cm)thick. Such tubesheets are typically made from carbon steel, stainlesssteel, nickel alloys, nickel-chromium alloys, nickel-molybdenum alloys,and the like. In such shell and tube heat exchangers, the heat exchangetubes are typically from 0.5 inches (1.2 cm) to 2 inches (5 cm) innominal diameter. The heat exchange tubes may be any length that allowsfor cooling of the effluent gasses or liquids. The length of the heatexchange tubes will vary depending on the heat exchanger design, thediameter of the tube, the coolant flow and the like. Thus, the tubelength for a particular reactor design is well within the ability of oneskilled in the art. Typical tube lengths are in the range of 4 feet (1.2m) to 30 feet (9 m). Tubes useful in shell and tube heat exchangers aretypically made from carbon steel, stainless steel, nickel alloy,nickel-chromium alloy, nickel-molybdenum alloy, and the like.

[0047] It is preferred that the heat exchange tubes are made from carbonsteel or nickel-chromium alloy. Suitable nickel-chromium alloys for usein heat exchange tubes contain 40 to 80% nickel and 12 to 28% chromium.The nickel-chromium alloys may optionally contain one or more othercomponents, such as carbon, silicon, manganese, copper, sulfur, cobalt,aluminum, iron, titanium, boron, phosphorus, molybdenum, or niobium.Suitable commercially available nickel-chromium alloys include thosemarketed under the INCONEL brand, available from Special MetalsCorporation (New Hartford, N.Y.). Suitable INCONEL alloys include, butare not limited to: INCONEL 600, INCONEL 601, INCONEL 617, INCONEL 625,INCONEL 718, INCONEL X-750, INCONEL 751, and INCONEL MA 754. It ispreferred that the nickel-chromium alloy used in the heat exchange tubescontain 71-75% nickel, 15-17% chromium, 7-11% iron, 0.2-0.35% manganese,0.2-0.35% silicon, 0.1-0.3 copper, 0.003-0.04% carbon, and 0.001-0.01sulfur %. Suitable preferred nickel-chromium alloy includes INCONEL 600.It is further preferred that the tubesheet and heat exchange tubes bemade from the same material.

[0048] The hot process gasses and/or fluids passing through a shell andtube heat exchanger may be cooled by any heat exchange medium. Suchmedium enters the shell through at least one inlet, passes along theheat exchange tubes, and exits the shell through at least one outlet.Suitable heat exchange media are any that remove heat from the processgasses and/or fluids. Suitable heat exchange media include, but are notlimited to: water, a mixture of water and steam, molten salt, glycol, amixture of water and glycol, oil, such as natural or synthetic oil,gasses, such as air and process gas streams, and the like.

[0049]FIG. 1a illustrates a typical weld used to affix heat exchangetubes to the tubesheets in shell and tube heat exchangers, such as thoseshown in FIGS. 3a to 3 e. FIG. 1a has the following elements: hotprocess gas inlet 1, heat exchange tube 2, weld 3, tubesheet 4, annularspace 5 and cooled process gas outlet 6. The annular space 5 extends upthrough the tubesheet and ends at the lower surface of the weld 3affixing the exchange tube 2 to the tubesheet 4. As the the coolingwater enters the annular space 5, it vaporizes, and deposits anyimpurities contained in the water in the annular space 5. Suchimpurities typically are ions and minerals in the water and the like.Such impurities are also typically corrosive to the tubesheet, exchangetube, and particularly the weld. Over time, such corrosive materialsbuildup in the annular space 5. The combination of heat from theeffluent gasses and the corrosive materials along with stresses in thesystem leads to stress corrosion cracking in the tube, weld and/ortubesheet. Heat treating the weld area minimizes the potential forstress corrosion cracking in the weld, particularly in nickel-chromiumalloy welds. However, such heat treating does not eliminate stresscorrosion cracking in the tubes and/or tubesheet. Such stress corrosioncracking leads to failure, and ultimately replacement, of the heatexchanger.

[0050] Other methods of affixing exchange tubes to tubesheets, such asrolling, are known. In rolling, an end of the heat exchange tube isrolled into a mating groove inside the tubesheet hole. FIG. 1b shows atypical rolled connection of an exchange tube to a tubesheet. Suchrolling methods are well known to those skilled in the art. When a tubeis affixed to a tubesheet by rolling, a small annular space 5 remainsbetween the exchange tube 2 and the tubesheet 4 which can also result instress corrosion cracking. Such rolling may be combined with welds asshown in FIG. 1a for additional mechanical strength. However, suchcombination does not eliminate stress corrosion cracking.

[0051] The heat exchange vessels used in reactors having reactive, hotprocess gasses, such as hydrogen cyanide reactors, have been designed tominimize such stress corrosion cracking. For example, each of the heatexchange vessels illustrated in FIGS. 3a to 3 d has a design thatfacilitates cooling of the effluent gasses. Each design provides forhigh cooling water flow rates, turbulent cooling water flow and optionalrefractory to insulate the upper tubesheet from the hot effluent gasses.By efficiently cooling the effluent gasses, these designs reduce therate of corrosion. However, these designs do not eliminate corrosion.

[0052] The upward flow orientation of the unit of FIG. 3e reduces theparticular problem noted above, that is, the concentration of corrosiveagents in the annular space, since hot water and vapor rise away fromthe tubesheet, and the exchange tube welds. However, this configurationstill suffers from the effect of deposits of minerals and otherprecipitants from the cooling water. These materials can also find theirway into the crevice between the tubesheet openings and the exchangetubes and cause damage to those components as well as the weld.

[0053] Therefore, known heat exchange vessel designs used in reactorshaving reactive, hot process gasses do not solve the problem of theconcentration of corrosive materials, and thus do not solve the problemof stress corrosion cracking.

[0054] The down-hole weld of the present invention greatly reducesstress corrosion cracking in heat exchangers having reactive, hotprocess gasses, such as hydrogen cyanide reactors, nitric acid wasterecovery exchangers and acrylonitrile reactors. FIG. 2a illustrates adown-hole weld useful in the present invention. FIG. 2a has thefollowing elements: hot process gas inlet 1, heat exchange tube 2, weld48, tubesheet 4 and cooled process gas outlet 6. In a down-hole weld,the top of a heat exchange tube is affixed to the lower side of atubesheet with a full-penetration weld. Thus, the annular space iseliminated and concentrations of corrosive materials are greatlyreduced. The use of down-hole welds greatly increases the service-lifeof shell and tube heat exchangers, particularly those used in hydrogencyanide production, by reducing stress corrosion cracking. The down-holeweld is preferably used in hydrogen cyanide reactors having heatexchange units as illustrated in FIGS. 3a to 3 e, and more preferably asillustrated in FIGS. 3a to 3 d.

[0055] The down-hole weld useful in the present invention may be formedby any conventional means, such as that described in U.S. Pat. No.4,221,263. It will be appreciated that the exchange tubes may be fittedin a variety of ways to the tubesheet for welding. For example, thetubes may fit into a counterbore or socket on the lower surface of thetubesheet and may be welded from either the inside of the tube to thetubesheet or on the outside of the tube to the lower surface of thetubesheet. See, for example, U.S. Pat. No. 4,221,263. It is preferredthat the down-hole weld used is that illustrated by FIG. 2a.

[0056] In preparing the preferred down-hole weld of the presentinvention, a hole having a diameter m is drilled almost completelythrough the tubesheet along an axis y, see FIG. 2b. A smaller holehaving a diameter n is then drilled through the remainder of the tubesheet along the axis y. The diameter n is sufficiently large that theexchange tube 2 can be inserted through the hole. The exchange tube isthen inserted into the tube sheet such that the exchange tube axis x andthe tubesheet hole axis y are coincident. The exchange tube 2 isinserted into the tubesheet hole a distance p from the lower face of thetubesheet. The distance p is equal to the length of the tubesheet holehaving the diameter n. The distance p may be any length that providessufficient area for welding the tube to the tubesheet. Typically, thedistance p is less than one-half the thickness of the tubesheet, andpreferably less than one-third the thickness of the tubesheet. Once thetube is inserted into the tubesheet, a full-penetration weld is formedbetween the tube and the tubesheet by any conventional means. It will beappreciated by one skilled in the art that the holes in the tubesheethaving diameters m and n may be made in one or more drilling steps.

[0057] The down-hole welds of the present invention are typically heattreated. In such heat treatments, the weld and surrounding metal areaare heated. The methods of heat treatment employed are those appropriateto the particular metals welded. Such heat treatment methods are wellknown to those skilled in the art.

[0058] Reactor components are also subject to destructive thermal,chemical and physical agents resulting from the various chemicalprocesses employed. Heat exchange tubes are particularly exposed to suchagents. For example, in hydrogen cyanide production, the resultinghydrogen cyanide gas produced must be cooled quickly in order tominimize degradation. In such reactors, the heat exchanger unit isplaced as close to the catalyst and reaction zone as possible. Thus, theupper portion of the heat exchange tubes, that is, that portion nearestthe catalyst and reaction zone, is continuously exposed to reactive,hot, effluent gasses having a reducing, carburizing and/or nitridizingenvironment. Optional refractory material, such as ceramic, is placed onthe upper surface of the tube sheet, that is, nearest the catalyst andreaction zone. Such optional refractory material insulates the tubesheetfrom the heat of the reaction. However, such optional refractorymaterial does not typically cover the heat exchange tubes, which arethus exposed to the heat and chemical species generated by the reactor.

[0059] One approach to increasing the service-life of heat exchangersexposed to such harsh environments is to insulate the heat exchangetubes, typically by placing ceramic ferrules through the tubesheet intothe upper end of the heat exchange tubes. Such ferrules typically onlyprotect the tubes from heat, but not necessarily from chemical andphysical agents. For example, ceramic ferrules of silica, alumina andzirconia are known to provide thermal protection. However, such ferrulesfail to provide adequate protection against chemical and physical agentsunder the harsh environments of hydrogen cyanide reactors, nitric acidwaste recovery exchangers, acrylonitrile reactors, tube-side firedboilers, tube-side fired exchangers or catalyst crackers. Under theseenvironments, the ferrules typically used, including known ceramicferrules, are sacrificial, meaning that they degrade and must bemonitored and replaced on a regular basis.

[0060] Another approach to increasing the service-life of heat exchangetubes exposed to such harsh environments is to make the heat exchangetubes from an alloy resistant to the reactor environment, such asnickel-chromium alloys. However, heat exchange tubes made from suchnickel-chromium alloys are still susceptible to such problems as metaldusting.

[0061] It has been surprisingly found that using ferrules includingnickel-chromium alloy or silicon nitride greatly increase theservice-life of shell and tube heat exchangers, particularly those usedin hydrogen cyanide production. It is preferred that the ferrules usefulin the present invention include silicon nitride.

[0062] Suitable nickel-chromium alloys for use in the ferrules of thepresent invention are those disclosed in U.S. Pat. No. 5,354,543, or anyother commercially available alloy. It is preferred that thenickel-chromium alloys useful in the present invention contain 40 to 80%nickel and 12 to 28% chromium. The nickel-chromium alloys may optionallycontain one or more other components, such as carbon, silicon,manganese, copper, sulfur, cobalt, aluminum, iron, titanium, boron,phosphorus, molybdenum, or niobium. Suitable commercially availablenickel-chromium alloys useful in the present invention include thosemarketed under the INCONEL brand, available from Special MetalsCorporation (New Hartford, N.Y.). Suitable INCONEL alloys useful in thepresent invention include, but are not limited to: INCONEL 600, INCONEL601, INCONEL 617, INCONEL 625, INCONEL 718, INCONEL X-750, INCONEL 751,and INCONEL MA 754. It is preferred that the nickel-chromium alloycontains 71-75% nickel, 15-17% chromium, 7-11% iron, 0.2-0.35%manganese, 0.2-0.35% silicon, 0.1-0.3 copper, 0.003-0.04% carbon, and0.001-0.01% sulfur. Suitable preferred nickel-chromium alloy includesINCONEL 600.

[0063] The silicon nitride ferrules useful in present invention are anythat include silicon nitride (Si₃N₄) or silicon nitride alloys. Suitablesilicon nitride materials useful in the ferrules of the presentinvention include, but are not limited to: silicon nitride, ceramicscontaining silicon nitride whiskers or silicon nitride alloys. Suitableceramics containing silicon nitride whiskers include, but are notlimited to: alumina, zirconia, and the like. Any silicon nitride alloyis suitable for use in the present invention. Such silicon nitridealloys include those containing up to 5% carbon, such as those disclosedin U.S. Pat. No. 4,036,653, herein incorporated by reference to theextent it teaches the preparation of such silicon nitride alloys. It ispreferred that the silicon nitride ferrule contains at least 95% siliconnitride (Si₃N₄), and more preferably at least 97% silicon nitride, andmost preferably at least 99% silicon nitride. It is further preferredthat the silicon nitride be hot pressed during manufacture.

[0064] The ferrules of the present invention may be any shape that fitsinto a heat exchange tube of a shell and tube heat exchanger. Thus, itwill be appreciated by one skilled in the art that the outer diameter ofthe ferrule is less than the inner diameter of a heat exchange tube. Theouter diameter of the ferrule may be anywhere from substantially thesame as the inner diameter of the exchange tube so as to provide a snugfit to significantly smaller than the inner diameter of the exchangetube so as to provide a very loose fit. One skilled in the art willeasily determine the ferrule outer diameter necessary for a given heatexchange tube design. By substantially the same is meant that an outerdiameter of a ferrule or any part thereof is small enough to fit withinan exchange tube or tubesheet hole while providing an effective sealbetween the ferrule or part thereof and the exchange tube or tubesheethole. The ferrules of the present invention typically have a nominaldiameter in the range of 0.5 to 2 inches (1.2 to 5 cm) and preferably0.75 to 1.75 inches (1.9 to 4.4 cm).

[0065] The ferrules of the present invention are inserted into heatexchange tubes such that the upper surface of the entry ends of theferrules, that is the ends closest to the reaction zone of the reactor,are at least flush with the upper surface of the entry tubesheet. It ispreferred that the entry ends of the ferrules of the present inventionextend above the upper surface of the tubesheet. It is further preferredthat the entry ends of the ferrules extend above the optional refractorylayer. When employed, such refractory layer is typically from 1 to 24inches (2.5 to 60 cm) thick. By extending the ferrule above the optionalrefractory layer, erosion of the optional refractory layer by the hoteffluent is reduced. Thus, the length of the ferrule is dependent uponthe heat exchanger design and the amount and type of refractorymaterial, if any, employed.

[0066] It will also be appreciated by one skilled in the art that theferrule be mounted or inserted into an exchange tube in such a way as toprevent the ferrule from sliding all the way down the tube, that is,away from where the protection it affords is needed. It is preferredthat the entry end of the ferrules of the present invention, that is theend closest to the reaction zone of the reactor, have a means forholding the ferrule in place. Suitable holding means include, but arenot limited to: lips, rims, ridges, flanges, flarings, clamps or thelike. It is preferred that the ferrules of the present invention have alip or a flared or conical entry end, and more preferably a flared orconical entry end. The ferrules of the present invention must have alength at least equal to the thickness of the entry tubesheet used inthe heat exchanger. Otherwise, the length of the ferrule is notcritical. If the ferrule is a sacrificial ferrule, meaning that it isexpected to be worn away during use, it is preferred that the ferrule belonger than necessary so as to prolong reactor run time before having toreplace the ferrule. The actual ferrule length is dependent upon theparticular effluent gasses in the reactor as well as the design of theheat exchanger, and such is within the skill of one in the art. It ispreferred that the ferrules of the present invention have sufficientlength to extend below the tubesheet. It will be appreciated that themore efficient the cooling medium is, the shorter the length of theferrule can be that extends below the tubesheet. It is preferred thatthe ferrule extends from 0.5 to 4 inches (1.2 to 10 cm) below thetubesheet.

[0067] Typically, the ferrules useful in the present invention have anoverall length in the range of from 1 to 30 inches (2.5 to 76 cm),preferably from 2 to 20 inches (5 to 50 cm).

[0068] The ferrules of the present invention may be used as is or may bewrapped with an additional insulation layer. Any fiber type materialwith low thermal conductivity is suitable for use as wrappinginsulation. Suitable insulation includes, but is not limited to:alumina, zirconia, silica, and the like. Such insulation may be in theform of a blanket, gauze, tape, and the like. For example, suitableinsulation includes silica paper, Fiberfrax®-Durablanket manufactured byUnifrax Corporation of Niagara Falls, N.Y., and Altra® RefractoryBlanket from Rath Performance Fibers, Inc., of Wilmington, Del. and thelike.

[0069] Suitable ferrule designs are illustrated in FIGS. 4 to 7. Theseferrules have the following elements which are identified by like numberin each figure: entry end 13, exit end 14, hot process gas inlet 15, andcooled process gas outlet 16. Arrows indicate the direction of gas flow.

[0070] Ferrules suitable for use in the present invention include thosehaving an insulating design. Such insulating ferrules have an entry endand an exit end;

[0071] the entry end having an opening tapering conically into a pipesection, the outer diameter of the entry end being greater than an innerdiameter of the heat exchange tube; the pipe section having an outerdiameter that is not more than 99% of the inner diameter of the heatexchange tube; the pipe section having an expanded area with an outerdiameter that is substantially the same as the inner diameter of theheat exchange tube.

[0072] Examples of alternate embodiments of insulating ferrules areillustrated in FIGS. 4a and 4 b. The ferrules of FIGS. 4a and 4 b have aconical entry end 13 having an outer diameter q which is generallylarger than the inner diameter of a heat exchange tube or thecorresponding tubesheet hole and thus holds the ferrule in place. Theferrules of FIGS. 4a and 4 b have pipe sections having a length t and anouter diameter r that is up to 99% of the inner diameter of the heatexchange tube. In FIG. 4a, the expanded area is the exit end 14 and hasan outer diameter s that is substantially the same as the inner diameterof the heat exchange tube so as to form an effective seal around theferrule when it is placed in a heat exchange tube. In FIG. 4b, theferrule has an expanded area 38 disposed between the pipe section andthe exit end 14. The expanded area 38 has an outer diameter s that issubstantially the same as the inner diameter of the heat exchange tubeso as to form an effective seal around the ferrule when it is placed ina heat exchange tube. In FIG. 4b, the exit end 14 is a distance d fromthe expanded area. Such distance d is not critical.

[0073] It will be appreciated by one skilled in the art that theexpanded area of the insulating ferrules of the present invention may bedisposed anywhere along the length of the ferrule. The closer theexpanded area is to the entry end, the shorter the insulating layer willbe. More than one expanded area may be disposed along the length of theferrule. The expanded areas of the insulating ferrules of the presentinvention may be bulbous projections, ridges, lips, flanges, flarings,and the like. When the expanded area is not at the exit end, it ispreferred that the end curves slightly inward, as shown in FIG. 4b. Suchinwardly curving end is particularly useful as an outer ferrule in atwo-ferrule system. Such expanded areas may be integral with the ferruleor may be separate components that are subsequently attached, such as bycementing, to the ferrules prior to being placed in an exchange tube.

[0074] Insulating ferrules of the present invention have the advantageof trapping gas in the annular space defined by ferrule exterior pipesection and the inner tube wall. Such trapped gas provides an insulationlayer to protect the heat exchange tube, and possibly the tubesheet holeand weld, from the heat of the process gasses. The width of the annularspace is equal to the difference between the outer diameter of theferrule pipe section r and the inner diameter of the heat exchange tube.It is preferred that the pipe section of the insulating ferrules of thepresent invention has an outer diameter in the range of 85 to 99% of theinner diameter of the heat exchange tube, and preferably in the range of90 to 98%. The insulating ferrules of the present invention may be anymetal, including bimetallics, ceramic or ceramic clad metal. Suitablemetals or ceramics include, but are not limited to: nickel-molybdenumalloy, nickel-chromium alloy, silicon nitride, zirconia, alumina, carbonsteel, 300 series stainless steel, 400 series stainless steel, monel andthe like. Preferred metals or ceramics include silicon nitride, carbonsteel or nickel-chromium alloy. If the outer surface of the ferrules ofthe present invention have a converging/diverging shape, it will beappreciated these may also provide an insulating annular space fortrapped gas, equivalent to that illustrated in FIGS. 4a and 4 b.

[0075] In another embodiment, the ferrules of FIGS. 4 and 4b may bewrapped with insulation along the pipe section t. Any fiber typematerial suitable as insulation for the ferrules as described above maybe used. Such wrapping may increase the insulating capabilities of theferrule.

[0076] The ferrule of FIG. 5 has a conical entry end 13 having an outerdiameter q which is generally larger than the inner diameter of a heatexchange tube or the corresponding tubesheet hole and thus holds theferrule in place. The pipe section of the ferrule has the same diameteras the exit end 14 which has an outer diameter s that is substantiallythe same as the inner diameter of the heat exchange tube. The innerdiameter u of the pipe section is uniform along the length of the pipesection. The silicon nitride ferrules useful in the present inventionand having the general shape illustrated in FIG. 5 are generallycommercially available as nozzles for sand blasting, such as fromCeradyne, Inc. (Costa Mesa, Calif.).

[0077]FIG. 6 shows one embodiment of a venturi shaped ferrule having aconical entry end 13 having an outer diameter q which is generallylarger than the inner diameter of a heat exchange tube or thecorresponding tubesheet hole and thus holds the ferrule in place. Thepipe section of the ferrule has the same outer diameter as the exit end14 which has an outer diameter s that is substantially the same as theinner diameter of the heat exchange tube. The inner diameter of the pipesection gradually increases from the base of the conical entry end tothe exit end 14. The inner geometry of the venturi-shaped ferrules ofthe present invention is that of a converging/diverging nozzle. In theseventuri-shaped ferrules, the entry end may be either conical ortrumpet-shaped, and preferably conical. In the venturi-shaped ferruleshaving a conical end, the cone angle is typically between 19 and 23degrees. The diverging angle in the venturi-shaped ferrules of thepresent invention is typically less than 30 degrees. The diverging angleis preferably between 5 and 7 degrees. Such venturi-shaped ferrules havethe advantage of minimizing tube wall erosion through reduced turbulencein the effluent gasses and/or liquids entering the heat exchange tubesas well as low pressure drop through the ferrule. The venturi-shapedferrules of the present invention may be made from any metal, includingbimetallics, ceramic or ceramic clad metal. Suitable metals or ceramicsinclude, but are not limited to: nickel-molybdenum alloy,nickel-chromium alloy, silicon nitride, zirconia, alumina, carbon steel,300 series stainless steel, 400 series stainless steel, monel and thelike. Preferred metals or ceramics include silicon nitride, carbon steelor nickel-chromium alloy,

[0078]FIG. 7 shows a ferrule having a straight bore and an outer lip 35.The inner diameter of the entry end 13 and the exit end 14 aresubstantially the same. The inner diameter of the ferrule issubstantially uniform along its length. The outer lip 35 provides ameans to hold the ferrule in place, either on top of the tubesheet or ontop of the refractory layer. Thus, such outer lip 35 has an outerdiameter that is larger than the inner diameter of the heat exchangetube employed. It will be appreciated that the outer lip 35 may beplaced anywhere along the length of the ferrule. However, such ferruledesigns are susceptible to fracture when subjected to thermal shock,such as those encountered in a hydrogen cyanide reactor.

[0079] The nickel-chromium alloy and silicon nitride ferrules of thepresent invention may effectively be used with tubesheets and tubes madefrom carbon steel, stainless steel, nickel alloys, nickel-chromiumalloys, nickel-molybdenum alloys, and the like. It is preferred that thenickel-chromium alloy ferrules of the present invention are used in heatexchangers having nickel-chromium alloy tubesheet and heat exchangetubes. The nickel-chromium ferrules of the present invention areeffective in providing chemical, physical and thermal protection to thetubes. The silicon nitride ferrules are very effective in providingthermal protection as well as chemical and physical protection to thetubesheet, welds and exchange tubes with virtually no degradation at allin all environments. The silicon nitride ferrules of the presentinvention are particularly effective in protecting tubesheets, tubes andwelds from chemical and physical degradation in reducing, carburizingand/or nitridizing environments, such as in hydrogen cyanide reactors.

[0080] In another embodiment, turbulators, also called twisted tape, maybe added to the ferrules of the present invention that do not have aventuri-shape in longitudinal cross-section. Such turbulators aretypically a separate element that slides into, or is otherwise insertedinto, the ferrules of the present invention and may extend past the exitof the ferrules into the tubes. Such turbulators impart a corkscrew flowpattern to the effluent gasses entering the heat exchanger. Such a flowpattern reduces the formation of a stagnant boundary layer of gas at thetube wall, thus improving the overall heat transfer of the exchangertube. Thus, the process gasses will be quenched more quickly. Suitableturbulators include helical and double-helical inserts. Such inserts maybe made of metal, such as carbon steel, stainless steel, nickel alloy,nickel-molybdenum alloy and nickel-chromium alloy.

[0081] In yet another embodiment, the ferrules of the present inventionmay be rifled. By rifling is meant that a helical groove, ridge or otherprotrusion is added to the inside of the ferrule. Such a ridge has theadvantage of imparting a corkscrew flow pattern to the effluent gassesentering the heat exchanger. Such rifling is typically in the form of ahelix or a double helix. Such rifling may be achieved by grinding,grooving, or the like the inside wall of the ferrules of the presentinvention. Alternatively, the ridge may be formed during the casting ofthe ferrule.

[0082] In still another embodiment, the ferrules are placed in sleevesprior to inserting the ferrules into the heat exchange tubes. Thesleeves for use with the ferrules of the present invention are typicallya short hollow cylinder, such as a section of exchanger tube. Suchsleeves have the advantage of holding the ferrules at a specified heightabove the tubesheet while optional castable refractory is installed.This allows the ferrule to extend to the top of or above the optionalrefractory material placed on the top of the tubesheet. When ferrulesleeves are used, the ferrule length must be increased to account forthe length of the ferrule above the tubesheet, so that the exit end ofthe ferrule extends to at least the bottom of the tubesheet. The sleevesmay be made of any material that can support the ferrule until therefractory material is put in place. Thus, the ferrule sleeves may beceramic, such as alumina, silica, zirconia, silicon nitride and ceramicreinforced with silicon nitride whiskers; metal, such as carbon steel,stainless steel, nickel alloys, nickel-chromium alloys andnickel-molybdenum alloys; wax; plastic, paper, cardboard and the like.It is preferred that the sleeve is ceramic or metal, and more preferablysilicon nitride or nickel-chromium alloy. Such sleeves need only supportthe ferrule until the refractory material is added to the top of thetubesheet. When pre-cast refractory is used, ferrule sleeves are notrequired.

[0083] In certain applications or reactor designs, it may be desirableto use a multiple-ferrule system, such as a two-ferrule system. Atwo-ferrule system includes an inner ferrule, typically chosen for itsinsulating capability and/or chemical resistance, and an outer ferrule,primarily chosen for its durability. Suitable inner ferrules includeceramics, such as alumina, zirconia, silicon nitride, alumina reinforcedwith silicon nitride whiskers, zirconia reinforced with silicon nitridewhiskers, and the like. It is preferred that the inner ferrule issilicon nitride. The outer ferrule may be made of any material. Suitableouter ferrule material includes, but is not limited to: carbon steel,stainless steel, nickel, nickel-chromium alloy, nickel-molybdenum alloy,silicon nitride, and the like. It is preferred that the outer ferrule iscarbon steel, stainless steel, nickel-chromium alloy and siliconnitride. It is preferred that the outer ferrule is an insulating ferruleof the present invention.

[0084]FIG. 9 illustrates a two-ferrule system having an inner ferrule 40disposed within an insulating outer ferrule 42 having an expanded area46. The two-ferrule system is supported by ferrule sleeve 31 and restsin an opening in the refractory layer 44. The two-ferrule system passesthrough an opening in the entry tubesheet 4 and enters the entry end ofexchange tube 2. Exchange tube 2 is attached to the upper surface of theentry tubesheet 4 by a conventional weld 3.

[0085] An advantage of silicon nitride ferrules is that they arechemically compatible with the atmospheres of the effluent gasses inmany chemical reactors. Further, the silicon nitride ferrules of thepresent invention can be used in systems where the effluent gasses aredecomposed by exposure to catalytic metals in the heat exchange tubes,for example, in certain hydrogen cyanide reactors. The silicon nitrideferrules of the present invention are particularly useful in reactorswhich may contain one or more of the following effluent gasses:hydrogen, nitrogen, oxides of nitrogen, oxygen, carbon monoxide, carbondioxide, ammonia, methane and other gaseous hydrocarbons.

[0086] Thus, the ferrules and/or down-hole weld of the present inventionare preferably used to extend the service-life of shell and tube heatexchangers used in hydrogen cyanide reactors, such as in the Andrussowor Degussa B-M-A processes, nitric acid waste heat recovery exchangers,acrylonitrile reactors, titanium dioxide reaction systems, ammoniareaction and/or boiler systems, phosphoric acid reaction systems,sulfuric acid reaction systems, ethylene quench exchangers, tube-sidefired boilers, tube-side fired exchangers, waste incinerators orcatalyst crackers. It is more preferred that the present invention isused to extend the service-life of shell and tube heat exchangers inhydrogen cyanide reactors, nitric acid waste recovery exchangers andacrylonitrile reactors, and most preferably in hydrogen cyanidereactors. Thus, the invention is well suited to reactors producinghydrogen cyanide by reacting hydrocarbon, ammonia and optionallyoxygen-containing gas in the presence of a platinum-containing catalyst.

[0087] In one embodiment, the present invention provides a heat exchangeapparatus having increased service life including (a) a shell having anentry tubesheet portion and an exit tubesheet portion, each tubesheethaving a plurality of holes, wherein the shell has at least one inletand one outlet for heat exchange medium; (b) a plurality of tubesdisposed within the shell wherein an entry end of each tube is affixedto the entry tubesheet and an exit end of each tube is affixed to theexit tubesheet such that an axis of the tube and an axis of an entry andexit tubesheet hole are coincident, wherein each tube entry end isaffixed to the entry tubesheet by a down-hole weld; and (c) a pluralityof ferrules, each ferrule having an entry end and an exit end extendingthrough an entry tubesheet hole into a tube wherein the exit end extendsbelow the entry tubesheet, the ferrule including silicon nitride ornickel-chromium alloy. It is preferred that the exchange tubes includenickel-chromium alloy. It is further preferred that the ferrule includessilicon nitride.

[0088] In another embodiment, the present invention provides a heatexchange apparatus having increased service life including (a) a shellhaving an entry tubesheet portion and an exit tubesheet portion, eachtubesheet having a plurality of holes, wherein the shell has at leastone inlet and one outlet for heat exchange medium; (b) a plurality oftubes disposed within the shell wherein an entry end of each tube isaffixed to the entry tubesheet and an exit end of each tube is affixedto the exit tubesheet such that an axis of the tube and an axis of anentry and exit tubesheet hole are coincident, wherein each tube entryend is affixed to the entry tubesheet by a down-hole weld; and (c) aplurality of ferrules, each ferrule having an entry end and an exit endextending through an entry tubesheet hole into a tube wherein the exitend extends below the entry tubesheet, the ferrule having aventuri-design in longitudinal cross-section and includes siliconnitride or nickel-chromium alloy. It is preferred that the exchangetubes include nickel-chromium alloy. It is further preferred that theferrule includes silicon nitride.

[0089] In yet another embodiment, the present invention provides a heatexchange apparatus having increased service life including (a) a shellhaving an entry tubesheet portion and an exit tubesheet portion, eachtubesheet having a plurality of holes, wherein the shell has at leastone inlet and one outlet for heat exchange medium; (b) a plurality oftubes disposed within the shell wherein an entry end of each tube isaffixed to the entry tubesheet and an exit end of each tube is affixedto the exit tubesheet such that an axis of the tube and an axis of anentry and exit tubesheet hole are coincident, wherein each tube entryend is affixed to the entry tubesheet by a down-hole weld; and (c) aplurality of ferrules, each ferrule having an entry end and an exit endextending through an entry tubesheet hole into a tube wherein the exitend extends below the entry tubesheet, wherein the ferrule has an entryend and an exit end; the entry end having an opening tapering conicallyor trumpet-shaped into a pipe section, the outer diameter of the entryend being greater than an inner diameter of the heat exchange tube; thepipe section having an outer diameter up to 99% of the inner diameter ofthe heat exchange tube; the exit end of the ferrule having an outerdiameter that is substantially the same as the inner diameter of theheat exchange tube; and wherein the ferrule includes silicon nitride ornickel-chromium alloy. It is preferred that the ferrule includes siliconnitride. It is also preferred that the ferrule is wrapped withinsulation. It is further preferred that the exchange tubes includenickel-chromium alloy.

[0090] In a typical system for the production of hydrogen cyanide,reaction gas including hydrocarbon(s), such as methane, ethane,methanol, and the like, ammonia and optionally an oxygen-containing gasare fed into a reactor and reacted in the presence of a catalyst, forexample, a platinum-containing catalyst, at a temperature in the rangefrom about 1000° to about 1400° C.

[0091] When the reaction includes an oxygen-containing gas, it ispreferred that catalyst is an ammoxidation catalyst. The reactants aregenerally heated to the reaction temperature in the presence of thecatalyst. The high-temperature effluent gasses, that is product gasses,contain the hydrogen cyanide product. However, as discussed above, theeffluent gasses must be quenched to bring the temperature below about600° C. in order to lessen the decomposition of the hydrogen cyanide.This is done by passing the effluent gasses out of the reaction zoneinto a heat exchange zone through one or more ferrules communicatingwith exchange tubes of the heat exchange zone. In the heat exchangezone, the heat of the effluent is transferred to the material of theexchange tubes and then to a heat exchange medium surrounding the outersurface of the exchange tubes, thereby lowering the effluent fluid to asuitable temperature and recapturing thermal energy of the system forfurther use in the reactor or elsewhere in the operations.

[0092] This process is illustrated in FIG. 8 which shows that reactants17 enter the reaction zone 18. In the reaction zone 18, the reactantscome into contact with the catalyst 19, for example heated platinummetal gauze catalyst, supported on one or more layers of catalystsupport, here layers 20 and 34. The catalyst support may be, forexample, a honeycomb shape with or without ridges or a foam with orwithout ridges. Suitable catalyst support material includes, but is notlimited to metallic support screen, pre-cast ceramic or refractory,cast-in-place refractory, ceramic foam, ceramic packing, silicon dioxide(silica—SiO₂), silicon carbide (SiC), silicon nitride (Si₃N₄), siliconboride, silicon boronitride, aluminum oxide (alumina—Al₂O₃),aluminosilicate (mullite—3Al₂O₃—2SiO₂), aluminoborosilicate,carbon-fiber, refractory fiber, zirconium oxide (ZrO2), yittrium oxide(Y₂O₃), calcium oxide (CaO), magnesium oxide (MgO), Cordite(MgO—Al₂O₃—SiO₂), or combinations thereof. The hot, reactive effluent,containing hydrogen cyanide, exits the reaction zone 18, entering theheat exchange zone 21 through ferrules 22. The hot, reactive effluentpasses through the ferrules 22, into the exchange tubes 2 which aresurrounded by heat exchange medium 24. While passing through theexchange tubes 2, the effluent gasses are rapidly cooled from atemperature in the range from about 1000° to about 1400° C. to atemperature below about 600° C. The cooled effluent gas 23 then passesout of the heat exchange zone 21 through the exit ends of exchange tubes2 and the hydrogen cyanide product is separated from the effluent streamthrough conventional means not shown.

[0093] As further shown in FIG. 8, the heat exchange zone 21 may becomprised of a refractory layer 27 with openings extending therethrough,which openings are aligned with openings in the entry tubesheet 4. Thetubesheet 4 forms a wall of the heat exchange vessel 26 which containsthe heat exchange medium 24. The heat exchange medium 24 is preferablywater or a mixture of water and steam, but may be other fluids, asdescribed above, suitable for absorbing energy transferred from theheated exchange tubes 2. The ferrules 22 rest in the openings in therefractory layer 27, pass through the openings in the entry tubesheet 4,and enter the entry ends of exchange tubes 2. Preferably, the openingsin the refractory layer 27 are shaped to complement the exterior form ofthe ferrule 22.

[0094] The ferrule 22 is preferably surrounded by a separable orintegral ferrule sleeve 31 positioned along the longitudinal axis of theferrule 22. By “integral” is meant that the sleeve may be formed as asingle piece of common material with, or fixedly attached to, theferrule. It is preferred that the ferrule sleeve 31 be a separatecomponent to accommodate any movement of the ferrule 22 relative to theferrule sleeve 31, for example due to differential expansion duringoperations, without creating undesirable stresses within the materials.The ferrule sleeve 31 serves as a physical buffer between the ferrule 22and the interior of the surrounding refractory layer 27. The ferrulesleeves 31 also may serve to position the ferrules 22 in the desiredorientation over the entry end of the exchange tubes 2. At installation,the exterior of ferrule 22 and the interior of ferrule sleeve 31 may becoated with a thin layer of wax to aid in later removal of the ferrule.

[0095] In a conventional configuration, included within the scope of theinvention, the entry ends of the exchange tubes 2 extend through theentry tubesheet 4 into the reactor vessel 33. In this embodiment, shownin FIG. 8, the exchange tubes 2 are affixed by a weld formed between theexterior of the exchange tube 2 and the upper surface of the entrytubesheet 4.

[0096] The following examples are presented to illustrate furthervarious aspects of the present invention, but are not intended to limitthe scope of the invention in any aspect.

EXAMPLE 1

[0097] A hydrogen cyanide reactor was constructed with a shell and tubeheat exchanger. The entry tubesheet and heat exchange tubes of the heatexchanger were prepared from a nickel-chromium alloy (INCONEL 600). Theentry end of the exchange tubes was flush with the upper surface of theentry tubesheet or the tube ends extended slightly beyond the uppersurface of the entry tubesheet. The exchange tubes were welded to theupper surface of the entry tubesheet. A mixture of silicon nitrideferrules and nickel-chromium alloy ferrules (INCONEL 600) were placed innickel-chromium alloy (INCONEL 600) ferrule sleeves and the ferruleswere then inserted in the entry end of the exchange tubes. Afterapproximately 4 months (about 2700 hours) of operation, with 15operating cycles, that is thermal shocks, and several high temperature(>1200° C.) operating periods, one silicon nitride and onenickel-chromium alloy ferrule were removed and inspected for wear. Eachoperating cycle consisted of heating the ferrules during light-off fromabout 150° to 500° C. up to about 1200° to 1400° C. in about 1 minuteand a later period of nitrogen quenching of the ferrules fromapproximately 1000° to 1400° C. to about 25° C.

[0098] The nickel-chromium ferrule was difficult to remove from theexchange tube and it showed appreciable length loss, approximately ½inch (1.27 cm) from the original length. There were also large amountsof carbon/nitrogen deposits on the surface of this ferrule. The ferrulewas physically trapped within the ferrule sleeve. This ferrule had alsoswelled from absorbing carbon/nitrogen, thus constricting the innerdiameter of the ferrule.

[0099] The silicon nitride ferrule was easily removed from the exchangetube and showed no length loss. Some metal cyanide deposits were notedon the outside exit end of the ferrule. There was no visible internalwear nor swelling on the interior of the silicon nitride ferrule. Thus,the inner diameter of the silicon nitride ferrule was unchanged.

[0100] It can thus be seen that either a nickel-chromium ferrule or asilicon nitride ferrule protect the exchange tubes of a shell and tubeheat exchanger from hot, reactive effluent gasses, particularly fromeffluent gasses in a hydrogen cyanide reactor.

EXAMPLE 2

[0101] The hydrogen cyanide reactor of Example 1 was operated for anadditional months. Again one silicon nitride and one adjacentnickel-chromium alloy ferrule were removed from the reactor andevaluated.

[0102] The nickel-chromium ferrule was difficult to remove from theexchange tube and it showed appreciable length loss, approximately 1inch (2.54 cm) from the original length. There were also large amountsof carbon/nitrogen deposits on the surface of this ferrule. The ferrulewas physically trapped within the sleeve. This ferrule had also swelledfrom absorbing carbon/nitrogen, thus constricting the inner diameter ofthe ferrule. Erosion patterns (wall thinning) were also visible on theinterior of the ferrule.

[0103] The silicon nitride ferrule was easily removed from the exchangetube and showed no length loss. Some corrosion deposits, likely metalcyanide deposits, were noted on the outside of the ferrule. A darkcarbon deposit was noted on the outside exit portion of the ferrule.Slight erosion occurred on the inlet ridge of the ferrule. There was novisible internal wear or swelling on the interior of the silicon nitrideferrule. Thus, the inner diameter of the silicon nitride ferrule wasunchanged.

[0104] It can thus be seen that either a nickel-chromium ferrule or asilicon nitride ferrule protect the exchange tubes of a shell and tubeheat exchanger from hot, reactive effluent gasses, particularly fromeffluent gasses in a hydrogen cyanide reactor.

EXAMPLE 3

[0105] The ferrules from Examples 1 and 2 were analyzed for weight lossand volume change. These results are reported in the Table below. Thepercentages reported were estimated by comparing the weight and volumeof the ferrules from Examples 1 and 2 with that of a corresponding newferrule. Months of Weight Volume Ferrule Operation Change (%) Change (%)Silicon Nitride 4 2 0 9 4-5  0 Nickel-chromium alloy 9 9-10 9-10

[0106] Thus it can be seen that the silicon nitride ferrules of thepresent invention are very effective at protecting heat exchange tubesfor long periods of time without any significant change in thedimensions of the ferrule.

Example 4—Comparative

[0107] In the hydrogen cyanide reactor of Example 1, 4 aluminum oxideferrules having the design shown in FIG. 5 were employed instead of thesilicon nitride or nickel-chromium alloy ferrules. Two of the ferruleshad a purity of 95% and the other 2 had a purity of 97%. After 4-5months of operation, all 4 alumina ferrules were removed and inspected.In each case, the ferrule cracked or fractured in the neck region, thatis, where the flared entry end met the pipe section. In some cases, thepipe section had completely separated from the flared section of theferrule.

What is claimed is:
 1. A heat exchange apparatus for use in a reducing,carburizing and/or nitridizing environment comprising: (a) a shellhaving an entry tubesheet portion and an exit tubesheet portion, eachtubesheet having a plurality of holes, wherein the shell has at leastone inlet and one outlet for heat exchange medium; (b) a plurality oftubes disposed within the shell wherein an entry end of each tube isaffixed to the entry tubesheet and an exit end of each tube is affixedto the exit tubesheet such that an axis of the tube and an axis of anentry and exit tubesheet hole are coincident; and (c) a plurality offerrules, each ferrule having an entry end and an exit end extendingthrough an entry tubesheet hole into a tube wherein the exit end extendsbelow the entry tubesheet, the ferrule comprising silicon nitride. 2.The apparatus of claim 1 wherein the tubes comprise carbon steel,stainless steel, nickel alloy, nickel-chromium alloy ornickel-molybdenum alloy.
 3. The apparatus of claim 2 wherein the tubescomprise carbon steel or nickel-chromium alloy.
 4. The apparatus ofclaim 2 wherein the nickel-chromium alloy comprises 40 to 80% nickel and12 to 28% chromium.
 5. The apparatus of claim 1 wherein the ferrule hasa converging/diverging design in longitudinal cross-section.
 6. A heatexchange apparatus comprising: (a) a shell having an entry tubesheetportion and an exit tubesheet portion, each tubesheet having a pluralityof holes, wherein the shell has at least one inlet and one outlet forheat exchange medium; (b) a plurality of tubes disposed within the shellwherein an entry end of each tube is affixed to the entry tubesheet andan exit end of each tube is affixed to the exit tubesheet such that anaxis of the tube and an axis of an entry and exit tubesheet hole arecoincident, each tube being formed of a metal including nickel-chromiumalloy; and (c) a plurality of ferrules, each ferrule having an entry endand an exit end extending through an entry tubesheet hole into a tubewherein the exit end extends below the entry tubesheet, the ferrulecomprising nickel-chromium alloy.
 7. The apparatus of claim 6 whereinthe ferrule nickel-chromium alloy comprises 40 to 80% nickel and 12 to28% chromium.
 8. The apparatus of claim 6 wherein the ferrule has anentry end and an exit end; the entry end having an opening taperingconically into a pipe section, the outer diameter of the entry end beinggreater than an inner diameter of the heat exchange tube; the pipesection having an outer diameter up to 99% of the inner diameter of theheat exchange tube; the exit end of the ferrule having an outer diameterthat is substantially the same as the inner diameter of the heatexchange tube.
 9. A ferrule for use in a heat exchange tube wherein theferrule has an entry end and an exit end; the entry end having anopening tapering conically into a pipe section, the outer diameter ofthe entry end being greater than an inner diameter of the heat exchangetube; the pipe section having an outer diameter that is not more than99% of the inner diameter of the heat exchange tube; the pipe sectionhaving an expanded area with an outer diameter that is substantially thesame as the inner diameter of the heat exchange tube.
 10. The ferrule ofclaim 9 wherein the ferrule comprises silicon nitride or nickel-chromiumalloy.
 11. The ferrule of claim 10 wherein the ferrule comprises siliconnitride.
 12. The ferrule of claim 10 wherein the nickel-chromium alloycomprises 40 to 80% nickel and 12 to 28% chromium.
 13. A ferrule for usein a heat exchange tube wherein the ferrule has an entry end and an exitend; the entry end having an opening tapering conically ortrumpet-shaped into a pipe section, the outer diameter of the entry endbeing greater than an inner diameter of the heat exchange tube; the pipesection having an outer diameter that is substantially the same as theinner diameter of the heat exchange tube; and wherein the ferrule has aconverging/diverging design in longitudinal cross-section.
 14. Theferrule of claim 13 wherein the ferrule comprises ceramic, carbon steel,stainless steel, nickel alloy, nickel-molybdenum alloy, ornickel-chromium alloy.
 15. The ferrule of claim 13 wherein the ferrulecomprises silicon nitride, nickel-chromium alloy, carbon steel orstainless steel.
 16. The ferrule of claim 14 wherein the nickel-chromiumalloy comprises 40 to 80% nickel and 12 to 28% chromium.
 17. A heatexchange apparatus for use in a hydrogen cyanide reactor including: (a)a shell having an entry tubesheet portion and an exit tubesheet portion,each tubesheet having a plurality of holes, wherein the shell has atleast one inlet and one outlet for heat exchange medium; and (b) aplurality of tubes disposed within the shell wherein an entry end ofeach tube is affixed to the entry tubesheet and an exit end of each tubeis affixed to the exit tubesheet such that an axis of the tube and anaxis of an entry and exit tubesheet hole are coincident, wherein eachtube entry end is affixed to the entry tubesheet by a down-hole weld.18. The apparatus of claim 17 wherein the tubes comprise carbon steel,stainless steel, nickel alloy, nickel-chromium alloy ornickel-molybdenum alloy.
 19. The apparatus of claim 18 wherein the tubescomprise carbon steel or nickel-chromium alloy.
 20. The apparatus ofclaim 18 wherein the nickel-chromium alloy comprises 40 to 80% nickeland 12 to 28% chromium.
 21. The apparatus of claim 17 wherein thedown-hole weld is heat treated.
 22. The apparatus of claim 17 furthercomprising a plurality of ferrules, each ferrule having an entry end andan exit end extending through an entry tubesheet hole into a tubewherein the exit end extends below the entry tubesheet.
 23. Theapparatus of claim 22 wherein the ferrule comprises silicon nitride ornickel-chromium alloy.
 24. The ferrule of claim 23 wherein thenickel-chromium alloy comprises 40 to 80% nickel and 12 to 28% chromium.25. An apparatus for preparing hydrogen cyanide by reacting hydrocarbon,ammonia and optionally oxygen-containing gas in the presence of aplatinum-containing catalyst at a temperature in the range from 1000° to1400° C., including a reaction zone, an optional refractory zone, and aheat exchange zone including: (a) a shell having an entry tubesheetportion and an exit tubesheet portion, each tubesheet having a pluralityof holes, wherein the shell has at least one inlet and one outlet forheat exchange medium; (b) a plurality of tubes disposed within the shellwherein an entry end of each tube is affixed to the entry tubesheet andan exit end of each tube is affixed to the exit tubesheet such that anaxis of the tube and an axis of an entry and exit tubesheet hole arecoincident; and (c) a plurality of ferrules, each ferrule having anentry end and an exit end extending through an entry tubesheet hole intoa tube wherein the exit end extends below the entry tubesheet, theferrule comprising silicon nitride.
 26. The apparatus of claim 25wherein the tubes comprise carbon steel, stainless steel, nickel alloy,nickel-chromium alloy or nickel-molybdenum alloy.
 27. The apparatus ofclaim 26 wherein the tubes comprise carbon steel or nickel-chromiumalloy.
 28. The apparatus of claim 27 wherein the nickel-chromium alloycomprises 40 to 80% nickel and 12 to 28% chromium.
 29. The apparatus ofclaim 25 wherein each entry tube end is affixed to the entry tubesheetby a down-hole weld.
 30. A process for preparing hydrogen cyanidecomprising the steps of: feeding reaction gas to a reactor, the reactiongas comprising hydrocarbon, ammonia and optionally an oxygen-containinggas; reacting the reaction gas in the presence of a catalyst to giveproduct gas; cooling the product gas in a heat exchange apparatuscomprising (a) a shell having an entry tubesheet portion and an exittubesheet portion, each tubesheet having a plurality of holes, whereinthe shell has at least one inlet and one outlet for heat exchangemedium; (b) a plurality of tubes disposed within the shell wherein anentry end of each tube is affixed to the entry tubesheet and an exit endof each tube is affixed to the exit tubesheet such that an axis of thetube and an axis of an entry and exit tubesheet hole are coincident; and(c) a plurality of ferrules, each ferrule having an entry end and anexit end extending through an entry tubesheet hole into a tube whereinthe exit end extends below the entry tubesheet, the ferrule comprisingsilicon nitride; and recovering hydrogen cyanide from the cooled productgas.